Study Of The Feature Of Longshore Current And Its Instability On Mild Beach Slope

Presently, experimental study on longshore current is maily about field experiment and steep slope experiment. In order to study whether the character of longshore current on mild slope is different from steep slope, this paper studies mean longshore current for mild slope and its instabilities through physical experiment and numerical study. Characteristics of wave height, wave setup, mean longshore current velocity and longshore current instability for mild slope are obtained, which are different from steep slope.The analysis gives the effects of different mean longshore current velocity distributions to the instability growth mode by the linear instability method for slope 1:100 and 1'.40. The characteristics of vortex caused by the instability of longshore current are analyzed by the nonlinear shear instability model. The dispersion coefficient in wave field is derived, which is applied to the numerical simulation of ink diffusion in flow field of longshore current.This paper makes a physical model experiment of longshore current under the action of different wave height, wave period, regular wave and irregular wave cases for flat slope of 1 100 and 1:40. The longshore uniformity of longshore current induced by passive flow circulation system is disscussed. The characteristics of wave height, wave setup and mean longshore current velocity corresponding to these wave cases are obtained:wave height is concave down trend after wave breaking for mild slope. The slower, the concave trend is more obvious. The shoreward growth trend of wave setup is gradually slow. The slower, the shoreward growth trend of wave setup is slower. The coast side in the mild slope of 1:100 cases is concave down trend and the coast side in the gentle mild slope of 1:40 cases is convex trend. In order to discuss the formation mechanism of the above characteristics of velocity profile, this paper does numerical simulation of mean longshore current by using the mean longshore current mode. The results show that the main factor influencing the velocity profile is the choice of the expression of the bottom friction. The velocity profile of longshore current for slope 1:100 can be simulated by using the flow bottom friction coefficient while for slope 1:40 can be simulated by using the wave bottom friction coefficient.In order to show the oscillating characteristics of flow velocity caused by the instability of longshore current, the physical experiment shows the fluctuation of velocity by continuous ink and records the results by CCD cameras. The characteristics of ink movement and the characteristics of votex movement caused by the instability of longshore current reflected by ink movement are given. The characteristic period of the instability of longshore current is detemined by the analysis of maximum entropy spectrum and the wave length can be displayed by the osilating wave length of ink.The different instability growth modes caused by the different profile characteristics of slope 1:100 and 1:40 are obtained by using the linear shear instability model. The instability of longshore current may exist frontshear and backshear modes for slope 1:100 and the instability of longshore current only exists backshear mode for slope 1:40. The wave period and wave length of instability obtained by analysis of linear shear instability compared with the experimental results.The nonlinear characteristics of the evolution of instability of longshore currents corresponding to 1:100 and 1:40 beach slope are studied by numerically solving the two-dimensional horizontal nearshore loop equation. The influences of slope, wave height and irregular wave on the instability of longshore current and ink diffusion are elaborated. Effect of these factors on the influences of votex movement caused by the instability of longshore current is specially discussed.The analytical expression of dispersion coefficient is derived by successive approximation for flow field composed of Stokes drift and first order wave orbital velocity. And the second order dispersion coefficient is illustrated for the first time. The effect of Stokes drift and first order wave orbital velocity on the dispersion coefficient in wave field is disscussed and the physical significance and the quantity of all kinds of dispersion coefficients are also disscussed.